The icon is always a panel of a selected wood. Not every wood is suitable for an icon. The iconographer usually selects a wood without a resin in it, it is seasoned, boiled in hot water to kill every residue of resin in it. The wood has to be perfect.
The wood for the icon is usually taken from the centre of the trunk of the tree because the lines are almost straight. If taken from the upper part of the trunk, the tendency is that when it dries it can crack. In fact, some old icons are cracked because the wood was taken from the outer part of the bark and not from the centre.
When the panel is ready, the iconographer leaves a border and creates a recess in the wood.
The reason is that the idea of the iconographer is that the panel is a recipient of a prayer. The whole icon is a prayer written in colour. An icon is a meditation written in colour on wood. A prayer follows every momen of the long preparation – it is a spiritual procedure to arrive to the last moment when the colour is applied.
In olden times, before applying the colour, the whole surface of the icon was covered in gold and then the colour was applied on top of it. Today this is traced and only the parts that are not coloured are in gold leaf. This is because gold is a symbol of divinity, God’s presence.
NOTE: A very good explanation of Rublev’s icon about the Trinity is found on http://www.holy-transfiguration.org/library_en/lord_trinity_rublev.html .